circumference

The curve which incloses a circle, ellipse,
or other closed, plane figure. In figures bounded by straight lines, such
as the triangle, square, or other type of polygon, the term perimeter
is used to designate the sum of all the bounding lines taken together.

The length of the circumference depends partly on the nature of the curve;
thus, that of the circle = 2πr = 2πd; and that of
the ellipse = 2π[1 - (1/2)2e2/1 - (1 ×
3/2 × 4)2e4/3 - (1 × 3 × 5/2
× 4 × 6)2e6/5 - ...], where a
is the semi-major axis, and e the eccentricity.